Study on the Equivalent Average Temperature Variation of the Coal Core during the Freeze Coring Process

In the freeze coring process, the core tube is subjected to cutting heat, frictional heat with the coal wall, and refrigerant action, which causes the temperature of the coal core to be different at different positions and at different times. The equivalent average temperature is proposed to represent the change law of the whole temperature of the coal core and to provide the temperature boundary condition for calculating gas loss. Relying on the self-developed simulation platform for the freezing response characteristics of gas-containing coal, a temperature change simulation test of the freezing core under different external heat conditions was carried out, and the freezing core heat transfer model was constructed with the help of COMSOL to analyze the coal core radial temperature changes during the freeze coring process. Because the drilling sampling time of the freeze coring process is short and there is a thermal isolation device between the drill bit and the core tube, the influence of cutting heat is ignored when the model is established, and only the coal core diameter is studied. The results show that the law of equivalent average temperature of the coal core with time is consistent with the experimental law, which is divided into three stages: rapid decline, slow decline, and relative stability. The temperature drop amplitude and rate of the equivalent average temperature of the coal core decrease with increasing external heat temperature. For example, when the external temperature is 60, 70, 80, and 90 °C, the limit temperatures of the equivalent average temperature of the coal core are -36.301, -30.358, -23.956, and -18.899 °C, respectively.